1. Field of the Invention
The invention relates generally to spark ignition devices for internal combustion engines and to their method of construction, and more particularly to spark plugs having an outer metal shell and a ceramic insulator received at least partially in the metal shell.
2. Related Art
A spark plug is a spark ignition device that extends into the combustion chamber of an internal combustion engine and produces a spark to ignite a mixture of air and fuel. As illustrated in FIG. 1, a conventional spark plug 1 typically has an outer metal shell 2, a ceramic insulator 3, which is at least partially received and captured in the shell 2, a metallic center electrode 4 extending partially through the insulator 3 to a firing tip 5, and a ground electrode 6 extending from the shell 2 to provide a spark gap 7 in conjunction with the firing tip 5 of the center electrode 4.
Some known problems exists with conventional spark plugs that diminish their useful life. One problem is generally referred to as “thermal breakdown.” Thermal breakdown occurs via a mechanism involving a dielectric breakdown of the ceramic material used to construct the insulator 3. Failure from dielectric breakdown occurs in the form of a physical puncture by an electrical arc through the ceramic insulator in response to a high electrical field. the thermal breakdown mechanism occurs when localized heating from a small leakage current lowers the electrical resistivity of the ceramic, causing additional leakage current and additional heating until thermal runaway results in a physical puncture of the insulator. One method to reduce the effects of thermal breakdown is to conduct heat away from the ceramic and prevent thermal runaway. In conventional spark plugs, heat is transmitted from an upper or proximate tip 8 of the electrode 4 that is in electrical communication with a terminal stud 9, through the metallic electrode 4 and through the ceramic material of the insulator 3 to the surrounding metal shell 2, which is in contact with an engine block. A main location for the transmission of the heat through the ceramic insulator 3 to the shell 2 is an interface between the insulator and the shell, commonly at a gasket 10, which is typically compressed between a small shoulder 11 of the insulator 3 and a mating shoulder 12 of the shell 2. The gasket 10 provides a relatively small contact patch, and thus, the heat from the electrical discharge proximate the gasket 10 can not be efficiently dissipated via conduction. Accordingly, thermal conduction between the insulator 3 and the shell 2 of the conventional spark plug 1 is generally insufficient to reduce the thermal dielectric breakdown of the ceramic insulator 3 in this region.
Another problem known to reduce the useful life of conventional spark plugs results from mechanical stresses placed on the ceramic insulator 3, which can result in mechanical failure of the spark plug, such as through premature fatigue cracks in the ceramic material of the insulator 3, which can in turn exacerbate the aforementioned thermal breakdown phenomenon. The mechanical stresses are directly associated with the manner in which the insulator 3 is assembled in the outer metal shell 2. Typically, the insulator 3 is compressed axially between the small lower shoulder 12 in the shell 2, with the intermediary gasket 10 being between the lower shoulder 12 and the small shoulder 11 of the insulator 3, and an upper folded, rolled, or otherwise turned shoulder 13 of the shell 2. This method of assembly, although useful, imparts an axially compressive force on the insulator 3, which in turn can result in stress fractures in the insulator, and ultimately failure of the spark plug 1.
Accordingly, there is a need for spark plugs that resist failure mechanisms due to thermal and mechanical affects, that are suited for use in current and future high temperature/high performance spark ignition devices, that are economical in manufacture and exhibit a long and useful life.